1. Field of the Invention
This invention relates generally to forced air heating, ventilating and air conditioning systems and more particularly to a filter unit used at the end of a duct in such a system. The invention is particularly suited for use in an evaporative cooling system.
2. Description of Related Art
Virtually all modern buildings, whether residential, commercial or industrial, are equipped with systems for maintaining the air inside the building at a comfortable temperature and humidity, and reasonably free from airborne contaminants. A substantial art and industry supporting the design, fabrication and installation of such systems has grown up. This is often identified as the heating, ventilating and air conditioning (HVAC) art and industry, the term HVAC comprehends heating, cooling, filtering and related treating of air inside the building. Many such systems incorporate the forced movement of air within the structure through the HVAC apparatus; this type of system may be termed a forced air system.
The present invention is most closely related to the residential segment of the HVAC industry, though it will be recognized that HVAC apparatus for commercial and industrial structures comprise many of the same features as residential apparatus, differing primarily in size.
Those skilled in the HVAC arts recognize that there are many types of HVAC apparatus in widespread use today; they will also recognize that although the following discussion is couched in terms of representative types of apparatus, it is applicable to many other types of apparatus.
In temperate climates, which will be arbitrarily defined for this discussion as having frequent daily summertime temperatures in excess of 80xc2x0 F. and frequent overnight wintertime low temperatures below 40xc2x0 F., one very common type of HVAC system comprises a central heating/cooling apparatus, with a system of supply ducts for distributing the heated or cooled air throughout the structure and a system of return ducts for collecting air from various portions of the structure and returning said air to the heating/cooling apparatus. A filter is typically incorporated in such a system; it is typically situated in a return duct immediately adjacent to the heating/cooling apparatus. As the air inside the structure is recirculated through the living space in the structure and the HVAC apparatus, this HVAC system is termed a closed-loop system.
One purpose of filter elements in HVAC systems is to keep dust and dirt inside the structure down to a manageable level. Another purpose is to reduce the likelihood of injuries to the lungs of occupants of the building, where such injuries may result from breathing dirty air. However, the effectiveness of any filter apparatus depends on its design, and whether it is properly maintained. Maintenance typically means replacement of disposable filter elements or cleaning of reusable filer elements. If the heating/cooling apparatus is situated in a basement or utility room, maintaining or cleaning the filter element is relatively easy. However, if the heating/cooling apparatus is situated in a crawl space or attic, access to a filter is very difficult.
The type of filter element most commonly used in residential HVAC systems is comprised of a batt of glass fibers supported by expanded or perforated metal screens and a paper frame. Such filter elements are inexpensive, but also ineffective. Such filter elements typically remove less than ten percent of airborne particulate matter from the air passing through the filter element. Even where access to the filter element in an HVAC system is reasonably convenient, all but the most conscientious occupants of the structure are likely to maintain the filter less frequently than should be done. Where access to the filter is difficult, maintenance may be nonexistent.
Another type of HVAC system is frequently employed in hot dry climates, which will be arbitrarily defined for this discussion as having frequent daily summertime temperatures in excess of 95xc2x0 F. and prevailing dew points below about 60xc2x0 F. This is an evaporative cooler, also known as a swamp cooler. In such an apparatus, ambient air is drawn from the exterior of the structure across constantly moistened evaporator elements. Water on the evaporator elements vaporizes, absorbing the requisite heat of vaporization from the moving air stream. If the dew point of the ambient air is low enough, the air stream can absorb enough moisture to reduce its temperature to a comfortable indoor temperature. Because the effectiveness of an evaporative cooler depends upon the low dew point of the incoming air, these systems are operated as open-loop systems. Air is discharged from the interior of the structure to the outside air through exhaust ducts or open windows.
Because evaporative coolers operate as open-loop systems, all air entering the living space of the structure will contain whatever dirt and dust may be present in the ambient outdoor air. Typically, in a hot, dry climate, the air is very dusty and likely to contain dead vegetative matter. Also, any solids dissolved in the water used to moisten the evaporator elements will add to the airborne particulate matter as the water is evaporated. In addition, bacteria and mold may grow in the HVAC apparatus, creating another class of undesirable contaminants in the moving air stream. For structures of similar size, the volume of air passing through a swamp cooler is generally several times that passing through a closed-loop HVAC system. All of these factors point to a greater need for effective filtration in an open-loop HVAC system than in a closed-loop system. As evaporative coolers are frequently situated on the roof of a structure, rendering access to a filter located near the cooling unit impossible, filters are generally omitted from such HVAC apparatus.
In summary, filters in closed-loop HVAC systems are ineffective and filters in open-loop HVAC systems are nonexistent. Recognition of the need for more effective and more easily maintained filters for both closed-loop and open-loop HVAC systems led to the present invention. However, recognition of that need was only one factor in the present invention. Another factor was the realization that occupants of a structure can most easily maintain the filters in the HVAC system of that structure if the filter is situated within the living space thereof. Yet another factor was the discovery that those skilled in the HVAC arts had not heretofore adequately addressed that need.
The most relevant prior art known to the inventors comprises three patents issued to Hodge (U.S. Pat. Nos. 5,525,145, 5,690,719 and 5,989,303) and patents issued to Mack et al (U.S. Pat. No. 5,944,860) and Liedl (U.S. Pat. No. 5,176,570). The Hodge ""145 and ""719 patents and the Liedl patent are particularly relevant, for they identify a most preferred location for a filter in an HVAC system, namely, over a duct end at the point where that duct is joined to the living space within a room. The Hodge patents also provide an extensive survey of the HVAC arts existing as of the filing dates thereof, 1993 and 1995, respectively. Accordingly, disclosures of the Hodge ""145 and ""719 patents are incorporated herein by reference. Liedl""s teachings differ from Hodge""s to the extent that the only location he teaches for his filter apparatus is over a return air duct end.
The Hodge ""303 and Mack et al patents disclose pleated or fanfold filter elements that are situated within the air distribution or collection ducts. While the latter two patents disclose filters installed near the duct end, those filters are actually installed within the duct, which creates considerable inconvenience in installing the filter apparatus and considerable difficulty in maintaining the filter elements. As for filtering effectiveness, a pleated filter element does have a larger filter surface area than a planar element, thereby reducing the restriction to the air passing theretrough, but at the expense of limiting the thickness of the filter element. While the thickness of the filter element, often in the form of a nonwoven fabric substance, is limited by the pleated configuration, the overall length of the filter assembly (measured in the direction of air flow) is necessarily increased by that configuration.
In his ""145 and ""719 patent, Hodge points out shortcomings in the prior art teachings of Anderson (U.S. Pat. No. 1,886,460), Wright (U.S. Pat. No. 1,694,089), Kung (U.S. Pat. No. 5,240,487), Tynan (U.S. Pat. No. 1,429,811) and Liedl. Both Anderson and Wright teach registers that must be removed for access to filters installed therebehind. Tynen teaches a register attachment that is integral with the register. Kung, like Hodge ""303 and Mack et al, teaches installing filters in the duct behind the register or grill, as well as a scent dispenser. In short, none of these patents addresses the need for easy access to filter elements, to conveniently maintain those filter elements.
Although Hodge teaches easy access to the filter apparatus and easy removal from the duct end for maintenance in his ""145 and ""719 patents, his teachings are not without their own shortcomings. For example, in his ""145 patent, he teaches securing the filter apparatus to an existing register or grille by using magnets attached to the frame of the filter apparatus; this necessarily implies that there must be a register, and it must be made of a ferromagnetic material. This teaching is inoperative with respect to aluminum and polymeric registers, both of which are widely used in the HVAC arts. He also teaches that the perimeter of the filter apparatus should correspond to the perimeter of the register, thereby ensuring that the cross sectional area of the filter element cannot be substantially larger than the opening in the register. This also implies that each filter apparatus be custom made, since overall dimensions of a given nominal register size vary considerably among the various manufacturers thereof. He teaches that airflow through the filter element must be maintained at a high velocity in order to achieve self-charging electrostatic behavior in the filter element. However, any filter element is essentially an obstruction in the HVAC system, and maintaining a high air velocity through a filter element will necessarily create a significant pressure drop in the stream of moving air. The pressure drop that is an inescapable consequence of Hodge""s teaching places a significant load on the blower in the HVAC system, and necessitates careful sealing of all the ducts in the system to control air leakage therefrom. Furthermore, this excessive pressure drop is contrary to manufacturers"" recommendations against high-density filters, which restrict air flow and possibly damage the HVAC equipment. This problem is particularly acute in heat pump HVAC systems. In this respect, the teaching and operation of his invention contradict the stated objects of his invention.
In his ""145 patent, Hodge also teaches the desirability of washing the filter element and the frame at the same time. This necessarily limits the materials chosen for components of his filter apparatus to those that are not damaged by washing. The limitation is more severe if the filter apparatus is washed in a dishwasher, as Hodge suggests. Further, washing the entire filter apparatus may cause water to become entrapped in the channel that is an essential element of his teaching; such trapped water could leak onto the wall, or be blown into living space in the structure, if the filter apparatus in reinstalled before it is fully dry.
In his ""719 patent, Hodge teaches attaching a support frame to the wall around the duct end. A filter apparatus is removably secured inside the support frame. As he teaches securing the support frame to the studs of the structure, it will necessarily be no longer than the distance between the studs. The filter frame may be smaller than the opening in a register that would be secured aver the duct end in the absence of a filter apparatus. The problem of pressure drop across the filter element is the same as in the filter apparatus described in the ""145 patent, and perhaps worse.
The teaching of the Hodge ""719 patent with respect to removing the filter elements and retaining grids for maintenance, and subsequently reinstalling them, may be inoperative. Hodge teaches that the filter elements and retaining grids must be carefully sized, and be sufficiently flexible to enable insertion into the channel of the filter frame. If these components are sufficiently flexible to be removed from the filter frame and reinstalled by hand, without factory assembly machinery, then they may well be so flexible as to be blown out of the channel in the filter frame by the stream of moving air, especially if the pressure drop across the filter is as great as is implied by the limited cross sectional area of the filter.
Liedl points out the desirability of placing a filter element in the living space of a structure, in order to facilitate filter maintenance. However, his apparatus is rather bulky and it requires a significant amount of disassembly to gain access to the filter. Further, his location of the filter apparatus, namely, over a return air duct end, renders it inoperative with respect to evaporative cooling systems. This is a serious limitation with respect to HVAC systems used in hot, dry climates.
Development of the present invention began with the identification of the shortcomings of various types of prior art air filter apparatus, as discussed in the preceding section of this specification. Once these shortcomings were identified, it was possible to establish design criteria, or objects of the invention, that avoid them. Those objects are listed hereinbelow.
It is an object of the present invention to provide a filter apparatus for deployment over a duct end in a forced air HVAC system such that the apparatus may be conveniently and easily installed over either a supply duct or a return duct.
It is another object of the present invention to provide a filter apparatus that may be employed with either open-loop or closed-loop forced air HVAC systems.
It is another object of the present invention to provide a filter apparatus for deployment over a duct end in a forced air HVAC system such that it may be installed over an existing register at the duct end, or directly over the duct end without any associated register.
It is another object of the present invention to provide a filter apparatus for a forced air HVAC system such that the air passage through the apparatus is larger than the cross sectional area of the duct over which it is installed, thereby substantially reducing the static pressure drop across the filter apparatus and improving the filtration of airborne particulates.
It is still another object of the present invention to provide a filter apparatus such that a plurality of filter elements may be accommodated, and such that such filter elements may be conveniently and easily removed for maintenance without removing the filter apparatus from its position over the duct end in an HVAC system.
It is yet another object of the present invention to provide a filter apparatus for deployment over a duct end in a forced air HVAC system such that aesthetically pleasing designs may be incorporated therein.
Upon reading the following descriptions of the present invention and studying the accompanying drawings, these and other objects of the present invention will become apparent to one having ordinary skill in the HVAC arts. The following descriptions and drawings are presented with the intent of illustrating the concepts of the present invention, without limiting the scope of the present invention.
Briefly, the present invention provides a filter apparatus comprising a frame that is conveniently and easily disposed over a duct end in a forced air HVAC system, a filter element that is removably disposed within said frame, means to achieve easy access to said filter element for easy maintenance and/or replacement, and incidental related items to support the principal components identified herein. Unlike many forms of prior art filter apparatus, the filter apparatus of the present invention is not limited in size to the duct end over which it is installed, or a register to which it might be secured. In a preferred embodiment of the present invention, the filter elements are significantly larger in cross-sectional area than either the duct or the openings in an intervening register. Accordingly, the filter apparatus described herein thus provides significantly lower pressure drop across the filter element than prior art filter apparatus while improving filtration efficiency.
The filter apparatus of the present invention is provided with an opening in the side of the filter frame, over which a movable cover is fitted, so that the filter element may be easily removed for maintenance or replacement. Removal of the filter element is so simple that it may be performed by someone with little or no knowledge of the HVAC arts.
The filter element may be comprised of multiple layers of filter material, including, for example, a coarse-mesh screen, a glass fiber batt filter element and a polymer foam filter element, all disposed so that the air passing through the filter apparatus must pass sequentially through each layer of filter material.
The filter apparatus of the present invention has been developed with particular attention to flexibility in both application and installation. It may be employed with either open-loop or closed-loop HVAC systems. It may be employed over the end of an air supply duct, or an air return duct. It may be installed directly over a duct end, by securing it to the wall surrounding the duct end. It may be installed by securing it directly to an existing register, using hook bolts and thumb nuts. Although it is preferably larger in cross-sectional area than an existing register, it could be sized to match the overall dimensions of a register.
Specific features of the filter apparatus of the present invention are detailed in the following Detailed Description of the Invention and the accompanying drawings. The preferred modes of the present invention are also described therein. Those having ordinary skill in the HVAC arts will recognize alternative means of accomplishing the objects of the present invention, all of which are deemed to be equivalent to and to fall within the scope of the present invention.